Potential associations between the levels of micro-organisms in saliva and dental caries prevalence and incidence for a population of U.S. adolescents were investigated. Initial findings indicated that there was a significant positive correlation between the levels of S. mutans and lactobacillus in saliva and the prevalence of dental caries. The initial caries prevalence for participants having low levels of these micro-organisms in saliva were 32% and 34% lower than those having low levels for S. mutans and lactobacilli, respectively. During the 1st 17 months of the study these groups of subjects developed 54% and 67% fewer new DMF surfaces than those with high levels, respectively; by the end of 36 months they developed 44% and 48% fewer DMF surfaces than those with low levels, respectively.